Enable buildpacks for python django guestbook sample

From cloud-code-vscode-internal created by [j-windsor](https://github.com/j-windsor): GoogleCloudPlatform/cloud-code-vscode-internal#2642

Because of the implementation of the k8s Job waiting to do django migrate, we need to set /bin/sh -c entrypoint in dockerfile so we can specify args: ["python manage.py migrate"] in the migration job and it will run successfully for both buildpacks and docker.

This is currently blocked by https://github.com/GoogleContainerTools/skaffold/issues/4397